Effect of martensite-austenite (MA) distribution on mechanical properties of inter-critical Reheated Coarse Grain heat affected zone in X80 linepipe steel

The tensile deformation behaviour of X80 linepipe steels are compared following Gleeble thermal simulation of multi-pass welding cycles. Two microstructures were simulated with two different cooling rates for the second thermal cycle. The initial heating to 1350 °C was applied to simulate a coarse grained heat affected zone, followed by reheating to 850 °C with subsequent cooling at one of two rates (2 and 10 °C/s). The difference in these cooling rates produced a variation in martensite-austenite (MA) size, distribution, and morphology, while the fraction of MA remained comparable. It has been observed that the specimen cooled at 10 °C/s exhibited higher tensile strength and ductility compared to cooling at 2 °C/s. The MA microconstituents formed at 2 °C/s were coarser, which tends to initiate voids more readily during tensile strain. These coarser MA grains were also surrounded by ferrite grains which exhibited higher local grain misorientations. This led to inhomogeneous deformation and enhanced dislocation activity during straining, accelerating void formation, and hampering ductility. In contrast, specimens cooled at 10 °C/s exhibited a more uniform local strain distribution with finer MA structures, and higher ductility. Numerical simulation confirmed the detrimental effect of coarser MA was associated with higher local stress concentrations compared to finer MA.